Staring at smartphones in the dark messes vision

​London: In a warning to those who sleep with their devices next to them, researchers have found two women who were affected by transient smartphone "blindness" - a condition where they went blind in one eye after gazing at smartphones in the dark.

The first patient - a 22-year-old woman in England - had a habit of gazing at her smartphone before falling asleep.

"She would lie on her left side and look at the screen primarily with her right eye. Her left eye was often covered by the pillow," www.npr.org reported on Thursday.

The other patient in her 40s had similar problems when she woke up before sunrise and checked the news on her smartphone before sitting up.

It had been going on for about a year, ever since she had injured her cornea. Around the same time, she bought a smartphone, the report added.

"They were looking at their smartphones and they just happened to have one eye covered because they were lying in bed," Omar Mahroo, ophthalmologist at Moorfields Eye Hospital in London and an author reported in a paper published in The New England Journal of Medicine.

"In both cases, nothing bad was going on," Mahroo said, adding that it is just that one retina was adapted to light and the other to dark.

"The retina is pretty amazing because it can adapt to lots of different light levels, probably better than any camera," he noted.

Retinas constantly adjust when someone leaves a room and enters a slightly dimmer room or goes inside after being outdoors.

But these two women experienced a rare scenario in which that change would actually be noticeable.

To get to the root of the problem, the researchers asked the two patients to view the smartphone with just the left eye and then just the right eye on separate occasions.

They realised that the eye going temporarily "blind" was always the one that was being used to look at the bright screen.

To confirm this further, Mahroo went in a dark room and with one eye covered, looked at a smartphone for 20 minutes before turning off the screen.

"It did actually feel quite strange," he said. "It would be very alarming if you didn't know what was going on."

After flashes of dim light, the retina that had been exposed to the screen took longer to adjust to new light settings.

According to Mahroo, several other patients have said they experienced concerning vision loss because of smartphone use.

"We don't know of any ill effects" as of now but it can be jarring, the authors noted.